Seismometers

The equipment used in the field to measure the ground motion is a device called a seismometer. The Pacific Northwest Seismograph network has over 200 seismometers in the field throughtout Washington and Oregon. There are also different kinds of seismometers out in the field as well. There are what are called strong motion seismometers used for measuring earthquakes greater than a 2.0 magnitude earthquake, and there are short period stations used for measuring smaller earthquakes and other various motions such as that caused by a rock fall. These short period stations are so sensitive that we can read when a person walks by one of them!

Short period stations

Here we can see what two kinds of short period seismometer look like. The one on the left is a bigger, heavier, and older device than the one one the right, however it is hardier than the one one the right. The one on the right is smaller and lighter, and is much easier to carry to a more primitive site, such as in the crater of Mt. Saint Helens. However, while they both have a different appearance, the design of the inside and the way each works is basically the same.

How They work

Seismometers measure the amount the ground shakes by creating an electrical current. The way this works is that there is a magnet inside the device that stays locked into place, and on the outside of the magnet is a copper coil that moves up and down any time there is movement (this is sometimes reversed, depending on the instrument).     When this copper coil moves past the magnet, electrical current is created. The amount of current created by this motion indicates how much movement there was. So essentually we are measuring the amount of current being created.

Strong motion stations

Strong motion stations are used to detect the stronger earthquakes that we might feel or that could cause damage. With the short period stations, there is a limited range of motion that the magnets can travel, only allowing us to measure up to a magnitude 2.0 earthquake. With a strong motion station however, we can accurately measure the largest potential earthquake for the Pacific Northwest, and that is up to just over a magnitude 9.0! These stations can give us more accurate data on larger earthquakes, data that was once cut off.

Installation

When we are installing these devices out in the field, we typically use a couple of different methods, depending on what it is we are trying to measure. One meathod is to dig a hole about four feet deep or so and about two feet in diameter. We then pour concrete into the bottom of the hole and allow it to dry. We then place the instrument onto the concrete and make the instrument as perfectly level as possible. We then hook up the instrument to its power supply and connect the data line and run the wires via conduits to the surface. The instrument is covered up with the dirt and the area is restored to like it was before we were there.

The other way of installing an instrument is by putting the instrument above ground. With this method we just pour the concrete in some sort of base, allow it to dry, and then put the instrument on that. The instrument is then protected by a casing which we place over it.

To learn more about seismometers, click here

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